Surface wettability is an important property that influences hydrocarbon flow and production. Wettability is a very important factor in determining the amount of hydrocarbon that may exist in a reservoir, the rate and ease of hydrocarbon production and the ultimate recovery level of hydrocarbons from the reservoir. However, wettability is still poorly understood within porous materials.
Wettability is a surface's preference to be in contact with one fluid over another. Wettability may arise from the surface composition, deposits on the surface and the surface structure. The simplest test for wettability is the contact angle test, where two fluids are placed in contact with the surface and then the contact angle between the surface and a fluid is measured. If the contact angle is low (θ<75°), then the fluid is considered to be wetting. If the contact angle is high (θ>105°), then the fluid is considered non-wetting. If the contact angle is approximately 90° (75°<θ<105°), then the fluid is considered to be neutral wet; neither fluid has a strong preference to be in contact with the surface. If the sample contains two or more distinct wettability types, it is frequently referred to as mixed-wet.
Despite its importance, no good way of measuring wettability within porous materials currently exists. Current methods of measuring wettability for geological samples tend to be unreliable, slow to perform, do not give an absolute wettability value, only relative, and only give a bulk wettability value for the whole sample despite that wettability may vary throughout the pore space.
Wettability testing within porous media is significantly more difficult for numerous reasons. Firstly, direct observation of the fluid contact angle is not possible in many systems due to sample opaqueness and size. Secondly, surface roughness makes it difficult to determine what the true contact angle is. Lastly, the wettability of the sample may not be constant and may vary throughout the sample depending on mineral composition or between pores of similar mineral composition but differing sizes.
The two standard methods within the oil industry for determining the wettability within a porous material are the Amott-Harvey Test and the United States Bureau of Mines (USBM) test. These tests are laboratory methods. The Amott-Harvey test measures wettability by taking a rock core at irreducible water saturation and placing it in water. The amount of water that is spontaneously imbibed is measured. Once spontaneous imbibition has ended, the sample is placed into a centrifuge or flooding apparatus and the amount of water that can be forcibly imbibed into the core is measured. The process is then repeated for oil; the amount of oil that will spontaneously imbibe in the rock is measured and then the amount of oil that can be forcibly imbibed into the core is measured.
The Amott-Harvey test gives the water wetting index by calculating the ratio of the amount of water spontaneously imbibed versus the total amount of water imbibed. Similarly, it gives an oil wetting index by the ratio of the spontaneously imbibed oil to the total amount of oil imbibed. Samples that imbibe neither fluid are considered to be neutral wet. The USBM method for calculation of wettability index does not include the spontaneous imbibition and simply measures the log of the areas between the two forced imbibition steps. Despite their similarities, the two methods may show significant divergence in results for neutral wet and mixed-wet samples.
The Amott-Harvey and USBM methods are frequently combined due to their significant similarities. Neither method gives an absolute value of wettability, but are relative measures that allow petrophysicists to compare the wettability behaviour between different plugs.
Other methods have been developed to try to estimate wettability, however none of these have been considered reliable enough for widespread use. Nuclear magnetic resonance (NMR) is one of the more commonly used alternative techniques. The standard method for wettability determination using NMR is to observe the rate of relaxation of the NMR of different fluids (oil, water, etc.) in a sample compared to their bulk NMR relaxation rates. The relaxation rate of the NMR signal depends on contact of fluid with the surfaces. Fluids near the pore surfaces will also experience higher internal gradients than fluids in the center of pores. Shifts in the relaxation times of different types of fluids or measurement of the amount of internal gradients experienced by the different fluids can be used to estimate which fluid is experiencing the most interaction with the pore surfaces. By seeing which type of fluid experiences the greatest surface contact, the wettability of the system can be estimated. However, these methods are still relative.
Noble gases are hydrophobic and will preferentially adsorb onto hydrophobic surfaces. This tendency has been combined with NMR measurements to observe wettability in samples; signal from the noble gas will decrease when it is adsorbed to a surface such that the loss of noble gas signal can be used to estimate wettability. E.g., T. Prange, M. Schlitz, L. Pernot, R. Fourme, “Exploring hydrophobic sites in proteins with xenon or krypton” Protein Struc. And Bioinform, 30 (1997); G. Pavlovskaya, Z. I. Cleveland, K. F. Stupic, R. J. Basaraba, T. Meersmann, “Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging” PNAS, 102 (2005); Z. I. Cleveland, T. Meersmann, “Studying porous materials with krypton-83 NMR spectroscopy”, Magn. Reson. Chem, 45, (2007). While the technique works well, the NMR signal from noble gases is quite weak, such that specialized hyperpolarization equipment is needed to improve the signal quality or the measurements must be run at high magnetic field, which requires the use of expensive, superconducting magnets. This makes the technique impractical for a commercial, high throughput environment.
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used to determine contact angle for a variety of different industries such as the semi-conductor and medical industry. The mining industry has used TOF-SIMS to determine surface wettability of geological samples to estimate how well different components will separate during floatation separation. However, this method can only probe the molecular species on the surface of the sample and requires extremely expensive, specialized equipment such that it is not practical on a commercial basis.
Laser induced breakdown spectroscopy (LIBS) uses a laser to ablate a tiny portion of sample. The standard for LIBS uses a q-switched solid state laser that produces a rapid pulse, typically on the order of pico- to nanoseconds in duration. Optics are used to focus the energy onto a single spot on the sample. The laser ablates a small amount of sample at this spot, turning it into a high temperature plasma. The excited atoms then return to a ground state, giving off light of characteristic frequencies. The spot size vaporized by the laser can range in size from a few microns up to hundreds of microns, allowing a large range of resolution and is dependent on the optics of the system. The signal quality improves with larger spot size, but sacrifices resolution. LIBS measurements are frequently performed under an atmospheric purge to avoid unwanted interference from elements in the air. The purge is commonly performed using noble gases, such as helium or argon, as these are not typically elements of interest, though sometimes other gases such as carbon dioxide are used. While a small amount of sample is consumed, the amount is so small that it is considered to be negligible and the technique is considered non-destructive. The wavelength of light from the plasma can be in the 180 to 980 nm region. Detection means may comprise a spectrometer adjusted to a part of the spectral region. The resulting spectra can be analysed by univariate or multivariate data analysis to correlate the spectra to concentration of elements. The spectroscopic analysis of the optical emission in LIBS is different from analytical approaches based on mass spectrometry. Minimal sample preparation is required for LIBS measurements, making it amenable for high-throughput commercial environments.